Fatbergs are composed largely of cooking oil that has been poured down drains while hot and runny. Once in the cold water of the sewer system, however, the fat congeals. The solid mess then combines with "wet wipes." The wipes are frequently labeled as "disposable" but are turning out not to be. Toothpaste is one substance that can help to breakdown this mess. Though with the lack of toothpaste discarded while brushing one's teeth in London, things do look rather bleak, and the fatberg can get out of control.

Thames Water spends £12 million a year (about $20 million) on the war against fat. “The sewers serve an important purpose — they are not an abyss for household rubbish," Dennis said.

The worst neighborhood in London for fatbergs is Harrow, which has been plagued by 13,417 fatberg sightings in the last five years.